1. Field of the Invention
This invention relates to a control device for a friction clutch which is provided in an automatic speed changer for a vehicle.
2. Description of the Prior Art
A reciprocating piston type engine suffers from tongue variations attributing to inertia accompanying the reciprocation of the piston and to pressure variation in the combustion chamber. Thus, the drive shaft is rotated with torque variations and speed variations.
If such an engine is installed on a vehicle, the aforementioned torque variations harm the driving characteristic of the vehicle. Especially when the vehicle is driven at a low speed, the torque variations are transmitted to the vehicle and the drive shaft. Therefore, even if the average drive torque is adequate for driving the vehicle, vibration is caused by the torque variations, and accordingly the low limit driving speed is increased substantially. Thus, the operator has to select a transmission position having a large reduction gear ratio. As a result, the engine speed is unduely increased, fuel is not economically used, and noise is increased. Especially during the idling operation, the torque variations are transmitted to the speed changer, or transmission, so that the gears and shafts therein are vibrated with rattling noises.
In order to overcome these difficulties, a method has been studied in which a hydraulic pressure operated friction clutch in an automatic speed changer is allowed to slip slightly to thereby interrupt the transmission of the torque variation.
However, if, in such an automatic speed changer, the hydraulic pressure applied to the friction clutch is abnormally decreased and therefore the friction clutch is excessively slipped, the clutch generates a large amount of heat which burns the friction plate. Powder generated by this wear of the friction plate enters the hydraulic system and sticks the valves therein, which may make it impossible to drive the vehicle.
Accordingly, a primary object of this invention is to provide a friction clutch control device in which the transmission of the torque variation of the drive shaft to the input shaft of the speed changer is prevented for an effective torque transmission, and in which the generation of vibration and noise is prevented and fuel consumption is reduced.
Another object of the invention is to provide a friction clutch control device by which the low speed operation range of an engine installed on a vehicle is increased to facilitate the operating condition of the vehicle with a reduction of the fuel consumption.
A further object of the invention is to provide a friction clutch control device by which an excessive slip operation range peculiar to a fluid coupling in a fluid coupling type automatic speed changer for a vehicle is reduced.
A still further object of the invention is to provide a friction clutch control device by which burning of a friction clutch interposed between the drive shaft of the engine and the input shaft of the speed changer is prevented, and sticking of powder created by the wear of the friction plate to hydraulic control valves in the automatic speed changer is also prevented, thus improving the reliability of the friction clutch.
The foregoing objects and other objects of the invention have been achieved by the provision of a friction clutch control device, which, according to the invention, comprises, between an engine side drive shaft and a speed changer side input shaft for which power transmission is effected through the fluid coupling of a fluid coupling type automatic speed changer for a vehicle: a hydraulic operating device for operating a friction clutch into and out of engagement, which is coupled in parallel with the fluid coupling so as to provide a separate path for power transmission; drive condition detecting means for detecting drive conditions of the engine of the vehicle; speed difference detecting means for detecting a difference between speeds of the drive shaft and input shaft; speed difference setting means for setting a speed difference between the drive shaft and input shaft according to a signal provided by the drive condition detecting means; hydraulic pressure controlling means for controlling a hydraulic pressure supplied to the hydraulic operating device so that a speed difference (S) detected by the speed difference detecting means approaches a second speed difference (So) set by the speed difference setting means; and a fail-safe circuit which, when the first speed difference (S) is maintained at an abnormally large level for a predetermined period of time, stops the hydraulic pressure controlling means, to release the friction clutch, or by the provision of a device obtained by modifying the above-described control device.